Activating Axes checkbox on the Armature tab ‣ Display panel,
will show local axes for each bone’s tip. The Y axis is always aligned along the bone,
oriented from root to tip. So, this is the “roll” axis of the bones.

Basically, a bone controls a geometry when vertices “follow” the bone. This is like how
the muscles and skin of your finger follow your finger-bone when you move a finger.

To do this, you have to define the strength of influences a bone has on a certain vertex.

The simplest way is to have each bone affecting those parts of the geometry that are within
a given range from it. This is called the envelope technique,
because each bone can control only the geometry “enveloped” by its own influence area.

If a bone is visualized as Envelope,
in Edit Mode and in Pose Mode you can see the area of influence,
which depends on:

The distance property and

the root’s radius and the tip’s radius.

Our armature in Envelope visualization, in Pose Mode.

All these influence parameters are further detailed in the skinning pages.